The present invention relates to a permanent magnet rotary motor.
Generally, there is known a permanent magnet rotary motor which has armatures to generate a shifting magnetic field at a stator and a plurality of magnetic poles composed of permanent magnets at a rotor, which act with the shifting magnetic field to generate an electromagnetic force. However, in the motor with the permanent magnets as described above, pulsating torque (cogging torque) and a thrust force are produced at the time of no-load operation. Such cogging torque hinders smooth rotation or reciprocation of the motor, thus causing vibration and speed variation of the motor. As an approach to reducing the cogging torque, there have been hitherto proposed formation of a slot with an oblique groove, hereinafter referred to as a skew, in a stator or a rotor, or use of a permanent magnet having the shape of a sector with the center of an inner circular arc thereof not aligned with the center of an outer circular arc thereof, hereinafter referred to as the permanent magnet of an eccentric type. However, formation of the skew may reduce the torque or reduce the productivity of the motor. Further, use of the permanent magnet of the eccentric type cannot increase the magnetic flux density of the motor, so that higher torque. density cannot be achieved.
Then, in order to address these problems, Japanese Patent Application Laid-Open Publication No. 146584/1999 discloses a permanent magnet rotary motor. In this motor, a pair of end surfaces of each of a plurality of permanent magnets that are located on both sides of each permanent magnet in the peripheral direction of a rotor and oppose to each other in the peripheral direction are so formed as to extend in the radial direction of the rotor. The magnetic surface of each permanent magnet is configured to include an arc surface and a pair of inclined surfaces. The arc surface is arranged in the central portion of the magnetic pole surface in the peripheral direction, and the pair of inclined surfaces are located on both sides of the arc surface in the peripheral direction and are inclined in such a manner that a gap between the inclined surface and the magnetic pole surface of the magnetic pole section spread toward the peripheral direction as leaving from the arc surface. The higher torque density and reduction of the cogging torque are thereby effectively achieved.
In the conventional permanent magnet rotary motor described in the Publication No. 146584/1999, by setting an open angle Am of each inclined surface and an open angle As of each magnetic pole section of a stator so that the relation of ( 1/10) As<Am<(¼) As is satisfied, the cogging torque is reduced. However, just by using the permanent magnet of a type in which both ends in the peripheral direction of each permanent magnet are inclined, and just by setting the relationship between the open angles Am and As as described above, there was a limitation of reducing the cogging torque though the torque can be maintained to be comparatively high.